This invention concerns validators for, and methods for validating, security documents, in particular security documents comprising an embedded security device which is at least partially exposed by one or more exposure windows. The security documents to be validated may be banknotes, cheques or similar documents of monetary value.
Banknote validators, or counterfeit detectors, of various forms are already known. Automatic banknote validators are used in machines which accent banknotes as a form of payment such as vending machines. Automatic validators use relatively sophisticated validation techniques, such as high resolution scanning of a banknote in visible light to produce a scanned image which is compared with that expected of a valid banknote.
Another type of banknote validator is that used to augment the ability of a human operator to detect counterfeits. One such device which is relatively simple and inexpensive is known from International Patent Application No. WO94/16412. The device measures the ultraviolet fluorescence and reflectance characteristics of a banknote. Excessive levels of fluorescence can be detected in a counterfeit banknote, upon which the validator signals to the operator visibly and/or audibly to alert the operator to the invalidity of the banknote.
Although such validators to be used by human operators are now in general usage, it would be desirable to provide a further test whereby counterfeit banknotes can be detected.
Banknotes contain various security devices which are designed to be reproduced only with extreme difficulty, and to offer an immediate means of recognizion of a valid banknote to the human eye. One such device is the security strip or thread which is incorporated into banknotes in a number of countries. The security thread, usually consisting of a metallised plastics strip, is embedded in a banknote in such a manner that the thread is at least partially exposed by a number of windows which are located at spaced locations in the paper substrate. As a result, when the banknote is seen in reflected light, the security strip is visible at a number of locations corresponding to the positions of the windows in the paper, and in transmitted light the whole length of the security thread is visible. Thus, although the surface printing on a banknote may be copied readily by modern colour photocopying techniques, the security thread provides a further defense against counterfeiters.
It is known to provide validators which detect the presence, or absence, of a security thread in a banknote. Unfortunately, counterfeiters can reproduce a security thread by various ingenious methods.
Banknotes which are provided with security threads may be produced in a number of different ways. EP-A-0 059 056 describes a method in which a cylinder mould is used. A web of security thread is wound around the cylinder and supported by raised portions on the cylinder such that when paper fibres are deposited on the mould to produce paper webs, windows are produced corresponding to the raised portions of the mould. Windows might also be provided by embedding a security thread between two separately formed sheets of paper which are wet laminated or dry laminated together (see for example EP-A-0-229 645). One or both of these sheets may be provided with apertures, or relatively thin regions, through which the security thread is exposed in the paper product.
It is to be noted that in a number of countries the windows, however formed, are of greater length than necessary merely to expose the security thread across its width. This is due to the fact that the location of the security strip in the banknote paper is gradually varied so as to meander across the windows. As a result, when the banknotes produced are stacked into bundles, the positions of the security thread in the bundled banknotes are not all aligned, and extreme thickness of the bundles corresponding to the location of the security threads is avoided.
In one aspect, the present invention provides a method of validating a document comprising an embedded security device which is partially exposed by one or more exposure windows, the method comprising the steps or inspecting the document in one or more regions adjacent said security device, and judging on the presence of said one or more exposure windows on the basis of said inspecting step to provide a validation signal.
The windows associated with the security thread of a banknote are a feature which is difficult to reproduce and therefore suitable to provide for the relatively simple but effective validation of banknotes and detection of counterfeits.
It is possible to judge on the presence of the one or more windows by mechanically detecting thickness variations across the document. However, such direct thickness sensing would require sensitive, and therefore relatively costly, equipment. Mechanical thickness sensing would also be difficult to employ for hand-held use. The inspecting step preferably comprises generating radiation, locating the document such that said radiation impinges on the document, and sensing radiation transmitted through the document in one or more regions adjacent said security device.
The sensed radiation preferably comprises infrared radiation. Although the windows may be sensed using other forms of radiation, such as visible light, the windows are highly visible in the infra-red region of the electromagnetic spectrum. The surface printed inks on a security document such as a banknote are generally transparent to infra-red radiation, whereas thickness or density variations cause readily detectable differences in the amount of radiation transmitted.
The presence of an exposure window in a document could theoretically be determined by sensing the intensity of radiation transmitted through a single region of the document, i.e. the window itself. However, a counterfeit could readily pass such a test and to improve reliability and effectiveness it is preferred that radiation transmitted through a first region of the document is sensed to produce a first output and radiation transmitted through a second region of the document is sensed to produce a second output, which outputs are analysed to judge on the presence of said one or more exposure windows. The validation signal may be provided when said first output indicates that the sensed radiation has passed through an exposure window and said second output indicates that the sensed radiation has not passed through an exposure window.
The outputs may be analysed in a number of ways which may be used alone or in combination. For instance, the analysis may involve taking a difference between the sensed outputs, or may involve taking a ratio of the outputs. The difference or the ratio may be subjected to predetermined criteria in order to determine whether a validation signal should be provided.
The analysis may also involve comparing at least one of said outputs with a predetermined reference value or a predetermined range of values during said analysis. Preferably, this analysis involves determining whether one of said outouts corresponds with that expected on detection of a valid window, and determining whether the other of said outputs corresponds with the detection of a valid region between, or outside, said one or more windows
In one method of the invention particularly, but not exclusively, pertaining to banknotes, the security device is a strip or thread, and said first and second regions through which the sensed radiation passes are spaced in a direction parallel to said security device.
In one embodiment, said first output is produced by a first sensor and said second output is produced by a second sensor.
In another embodiment, said inspecting step comprises moving a sensor relative to said document in a direction generally parallel to said security device, and said first and second outputs are produced sequentially during the movement. This reduces the number of sensors required in order to determine the presence or absence of the exposure windows in the security document.
The inspecting step may comprise inspecting said document along a line to produce a profile of the document along said line, said validation signal being provided in response to, or in the absence of, the detection of a desired variation in said profile indicating the presence of said one or more exposure windows. The profile is preferably that of the transmission characteristics of said document along said line or inspection. The detection preferably involves detecting one or more windowed regions, and detecting one or more non-windowed regions. Preferably, the profile is analysed to perform said judging, said analysis involving determining the geometric relationship of said detected windowed regions and said detected non-windowed regions along said line of inspection. Although the analysis may involve determining a periodic relationship between the two regions corresponding to the regular spacing of the windows, that may require an undue degree of processing. In a less complex method the analysis may involve taking a ratio of the length of one or more of the detected windowed regions along said line with the length of more or more of the detected non-windowed regions along said line.
Since the security device itself, such as in the case of the security thread of a banknote, may be substantially opaque, it may be that i the document were inspected in only one region, the security device itself would obscure the exposure windows in transmitted light. Accordingly, it is preferred that the inspecting step comprises inspecting the document simultaneously in at least two regions, said judging stem comprising judging on the presence of said one or more exposure windows in any of said at least two regions. When the security device is a thread, those at least two regions are preferably spaced in a direction transverse to the length of the thread.
The present invention also provides apparatus comprising means for performing the method of the invention. In one embodiment, such means may comprise one or more radiation emitting means and one or more radiation detecting means, at least one of said emitting means preferably being located on a first support means and at least one of said detecting means being opposedly located on a second support means, said first and second support means being separated by an opening into which a document is to be inserted by a human operator for inspection using the apparatus.
To reduce complexity and cost, the sensor pairs are preferably small in number (these may be less than ten) and arranged to inspect only a portion of the document. The apparatus may further comprise means for indicating to a human operator a desired disposition of the security device during inspection. This should ensure reliable sensing of the exposure windows, which are located in the area of the security device.
According to a further aspect of the invention, a validator for security documents is provided, said validator comprising sensor means for discriminating between valid and counterfeit security documents, switch means for activating said sensor means, and support means for said sensor means, wherein said support means comprise a first portion and a second portion separated by an opening for a document to be validated, said first portion being movable with respect to said second portion to actuate said switch means.
Said sensor means preferably comprises radiation emitting means located on said first portion of the support means, and radiation sensing means located on said second portion of the support means.
Said radiation sensing means may comprise an infra-red radiation sensor.
The validator preferably comprises a plurality of radiation sensors providing a plurality or outputs to a processing means for analysing said plurality of outputs to provide a validation signal.
Said processing means preferably analyses two or more of said outputs in combination to determine whether said validation signal should be given.
The validator is preferably adapted so that when said support means is in said second relative position, a said sensor means may be moved relative to a document to be validated located between said first and second portions of the support means.
In said second relative position, said first and second portions preferably remain separated by an opening allowing a document to be validated to be moved relative to said sensor means.